Electrical Engineering ⇒ Topic : A.c series motor and its working principle
A.C. SERIES MOTOR
The series motor due to its desirable speed-torque characteristics is almost exclusively use in railway service. While the D.C. motor is entirely satisfactory for this class of work service and generally used on street railway cars and trolley coaches, the fact that it is more convenient and more economical to transmit power and to transform voltages in A.C. systems than with direct current has lead to the development of the A.C. series motor for use on some of the important stream-roas electrifications.
Working Principle. The working principle of an A.C. series motor is the same as that of the D.C. series motor. The armature and field are wound and interconnected in the same manner as D.C. series motor.
Fig.(a)Working principle of the A.C. series motor
When an alternating e.m.f. is applied to the terminals, since field and armature windings are connected in series, the field flux and armature current reverse simultaneously every half cycle, but the direction of the torque remains unchanged. The torque is pulsating, but its average value is equal to that which a D.C. motor will develop if it had the same r.m.s. value of flux and current. Motor connections, direction of torque, etc. for two successive half cycles are shown in Fig.(a). If the field and armature core are run at low saturation, the air-gap flux is approximately proportional to the current and the torque is approximately proportional to current squared.
Although it is theoretically possible to operate a D.C. series motor from an A.C. circuit, the following structural changes must be made in the motor to make it a practical and reasonable efficient machine :
Fig.(B) Connections for (a) conductive compensation (b) inductive compensation
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